Voice acting in these games is even worse than music is most of the times. I've never understood why people care about having these games voice acted.
I think it's about immersion. For instance, some might say they don't understand why porn needs a story, but we know that a half-decent story can enhance immersion and our overall enjoyment. Similarly, music can help set the tone of a scene, and voices can make the characters seem more real. (And it's worth mentioning that men can be aroused by a woman's voice, especially during sex.)
But the voices have to be GOOD. If they're poorly done or don't seem to fit the characters, then the voices become detrimental to immersion, and it's better not to have them. When considering how hard it is to do voices right, and the consequences of doing voices wrong, it's easy to see why devs usually don't bother taking the risk.
One example of a game that has some pretty good voices is
FreshWomen. The scenes with Susan in the last update were great, and her voice was like the icing on the cake for me (but obviously mileage may vary).
There's also the risk of a character's voice clashing with your idea of them, especially if voices are added later in development and not implemented from the beginning. To be honest, I actually found the moaning in the Dream Scenes a bit off-putting, because it didn't sound like how I had imagined the characters would sound, if that makes sense.